These studies have been directed at answering three general questions: 1) definition of the contribution of cell-mediated immune mechanisms to resistance of humans to certain opportunistic infections; 2) evaluation of immunologic reconstitution as a therapeutic tool in management of patients with certain opportunistic infections; and 3) identification and characterization of the components of dialyzable transfer factor. The results obtained show that approximately 66% of patients with chronic mucocutaneous candidiasis have definable defects in cellular immunity. In most cases, these defects can be corrected by treatment with transfer factor. Correction of the defect alone is not associated with clinical benefits. However, if the infections are cleared with antibiotics and transfer factor is used as a form of consolidation therapy, sustained remissions are observed. Transfer factor contains many components some of which mediate antigen-dependent effects while others appear to serve as modulators of the antigen-dependent reactions.